Precipitation hardening stainless steel

ABSTRACT

Martensitic precipitation hardened stainless steel consisting of 16-19% by weight of Cr, 5-10% by weight of Ni, 0.15-3% by weight of Nb, 0.1-1.5% by weight of Al, not more than 0.1% by weight of C, not more than 1.5% by weight of Mn, not more than 1.4% by weight of Si and the remainder being Fe and unavoidable impurities has a spring bending limit at least 1.5 times as high as that of the conventional 17-7 PH steel, and the stainless steel having 0.3-3% by weight of Nb and 0.1-1.0% by weight of Al particularly among said composition range is very suitable as a spring stainless steel having a good workability.

United States Patent [191 Hirayama et al.

[in 3,840,366 451 Oct. 8, 1 974 1 1 PRECIPITATION HARDENING STAINLESS.STEEL [73] Assignees: Hitachi, Ltd.; Hitachi Metals, Ltd., both ofTokyo, Japan; part interest to each [22 Filed: May 24,1972

211 Appl.No.: 256,274

Related US. Application Data [63] Continuation-in-part of Ser. No.830,619, June 5,

1969, abandoned.

[30] I I Foreign Application Priority Data Schatmeister...-., 75/128 (12,531,155 11/1950 Philips 75/128 G 2,758,025 2/1956 Clarke 75/128 G3,300,347 l/1967 Kasza 75/128 G 3,347,663 10/1967 Bieber 75/124 OTHERPUBLICATIONS Metals Handbook by American Society to Metals Vol. 1 (1961)pages 484 and 485.

Primary Examiner-Hylancl Bizot Attorney, Agent, or FirmCraig '&Antonelli [5 7] ABSTRACT Martensitic precipitation hardened stainlesssteel consisting of 16-19% by weight of Cr, 5-10% by weight of Ni,0.15-3% by weight of Nb,- 0.l-1.5% by vvweight of A1, not more than 0.1%by weight of C, not more than 1.5% by weight of Mn, not more than' 1.4%by weight of Si and the remainder being Fe and unavoidable impuritieshas a spring bending limit at least 1.5 times as high asthat of theconventional 17-7 PH steel, and the stainless steel having O.'3 -3% byweight of Nb and 0.1-1 .0% byv weight of Al particularly among saidcomposition range is very suitable as a spring stainless steel having agood workability.

8 C1aims, 3 Drawing Figures June 10, 1968 Japan 43-39414 [52] US. Cl. 75/124, 75/128 G, 75/128 T [51] Int. Cl. C22c 37/10, C22c 39/20 [58]Field of Search 75/128 G, 124

[56] References Cited UNITED STATES PATENTS 2,159,497 5/1939 Becket75/128 G l /5 l ,4 l l PATENTEDUBT a 1974 Gr (wt PRECIPITATION HARDENINGSTAINLESS STEEL This is a continuation-in-part of our copendingapplication, Ser. No. 830619 filed on June 5, 1969 now abandoned.

This invention relates to a martensitic precipitation hardeningstainless steel, and particularly to a precipitation hardening stainlesssteel which has a substantially austenitic structure or a considerablylarge amount of retained austenitic structure at annealing after asolution heat treatment and said structure is capable of being convertedto a substantially martensitic structure by applying a predeterminedcold work to the annealed stainless steel. 1

In the present invention, a stainless steel having good characteristicsparticularly for a spring material is to be obtained.

Precipitation hardening stainless'steel is a stainless steel recentlydeveloped by adding Al, Ti, Cu, Mo and the like metals to a 17Cr 7Nistainless steel, which has been heretofore known as a strong stainlesssteel capable of being used directly after the cold working, to impart aprecipitation hardenability to the stainless steel as well as to produceproperties suitable for a wide range of application fields.

The precipitation hardening stainless steels can be classified into thefollowing three types according to the structure and the heattreatability.

l. Martensite type A blending composition of components, which tends toeffect transformation from austenite to martensite, is selected, and aprecipitate, which is soluble in austenite structure but insoluble inmargensite structure after the transformation, is precipitated in themartensitic matrix.

It is known that there are two kinds of the martensite type stainlesssteel, that is, the stainless steel (17 Cr 4 Ni stainless steel, usuallycalled 17-4 PH), in which the transformation from the austenitestructure to the martensite structure takes place immediately during acooling step after the solution heat treatment and in which theprecipitation hardening can be effected by applying one heat treatmentto the transformed stainless steel, and the stainless steel (17 Cr 7Nistainless steel, usually called 17-7 PH), in which all or the most ofthe structure remain austenitic even after the steel is cooled only bythe solution heat treatment. This austenitic structure can betransformed to the martensitic structure only by applying anintermediate heat treatment to the steel or by cold working, and theprecipitation hardening is capable of being effected by applying a heattreatment to the steel.

2. Austenitic type A stainless steel in which the hardening is effectedby precipitating compounds rarely solid-soluble in the austeniticstructure. A 17 Cr 10 Ni stainless steel, that is, a stainless steelusually called 17-10P, is known.

3. Austenite-ferritic type 1 A stainless steel having a two-phasestructure, that is, austenite and ferrite, in which compoundssolid-soluble in the austenite but solid-insoluble in the ferrite areprecipitated from the ferrite phase.

Among these precipitation-hardening stainless steels, both 17-7 PH and17-4 PH are most widely used, but a 17-7 PH steel which has a highspring bending limit is particularly excellent as a spring material.

The term spring bending limit used in the present invention is definedas a surface stress of a sheet metal 2 beam, whose span length (l) isgiven by an equation I 8,000 ft or I 4,000 h, wherein It is a thicknessof the beam, when the residual strain at the center of the beamreaches-0.05 or 0.025 mm. I

Said spring bending limit" represents a degree of resistance to theso-called relaxation of the spring. The

vgree of cold working.

17-7 PH steel having a nominal chemical composition (C 0.09% by weight;Si 1.0% by weight; Mn" 1.0% by weight; P 0.04% by weight; S 0.03% byweight; Ni: 6.50-7.75% by weight; Cr: 16-18% by weight; Al: 0.75-l.5% byweight; and'Fe: the balance) has, when precipitation-hardened at a 50%reduction, a spring bending limit of about kg/mm Further, a 17-7PHstainless steel having said nominal chemical composition, to whichvery small amounts of Ti, Zr, U and the like are added to improve theweldability of said 17-7 PI-Iv steel, or a 17-7 stainless steel. inwhich 4% by weight or less of Cr is replaced with the same proportion ofMo to increase the high temperature strength, are known.

The amounts of Ni and Cr in the Ni-Al compoundprecipitation typestainless steel are selected from wide range compositions, dependingon-the service purpose, and the ranges which will ensure such excellentprecipitation hardenability are 5-10% by weight of Niand l6- -l9% byweight of Cr.

An object of the present'invention is to obtain a stainless steel havinga much higher spring bending limit by improving the conventional 17-7 PHstainless 'steel as a base.

Another object of the present invention is to obtain a stainless steelhaving a good workability and a high spring bending limit. i

The stainless steel ofthe present invention consists of 16-19% by weightof Cr, 5-10% by weight of Ni, O.15-3% by weight of Nb, 0.l1.5% by weightof Al, and the balance being Fe and unavoidable impurities. Further, itis not objectionable to replace a portion of Cr with Mo and a portion ofNb with Ti. H

The present invention will be hereunder explained in detail withreference to the accompanying drawings and Examples. I

FIG. I- is a graph showing a range for a preferable mixing proportion ofCr and Ni according to the present invention;

FIG. 2 shows a block diagram of precipitation hardening treatment stepsof the present stainless steel; and

FIG. 3 shows a graph of characteristic curves of the hardness and springbending limit of one embodiment of the present stainless steel. I

The present stainless steel has an austenitic structure or aconsiderable'amount of retained austenitic structures when the steel issubjected to a solution heat treatment and then cooled directly as itis, as explained above, and has a properly selected, specific mixingproportion of Cr and Ni so that such a structure may be converted tosubstantially martensitic structure by applying thereto a cold workingat a predetermined de- FIG. 1 shows a correlation of mixing proportionsof Cr and Ni which are effective for carrying out the pres ent inventionwithin said range. The hatched area in FIG. 1 is the most effectivezone. In the zone where the amounts of Cr and Ni are larger than in thehatched zone, the hardening action by the precipitation hardeningtreatment becomes weak due to an excessive stabilization, and in thezone where the amounts of these two components are less than in thehatched zone, the hardening tends to take place, on the contrary,immediately after the solution heat treatment, and consequently theworkability becomes poor. Furthermore, in the zone where the amount ofCr is less than 16% by weight, the corrosion resistance becomes ratherpoor. A sufficiently satisfactory result cannot be obtained in any ofthese cases. 3

As a method for the precipitation hardening treatment of l7-7 PH steelas in the present invention, three procedures, that is, TH procedure, RH(subzero) procedure and CH procedure, are known according to the mannerfor transforming the structure of steel after the solution heattreatment to the martensitic structure.

FIG 2 shows a typical block diagram when the precipitation hardeningtreatment is effected according to said three procedures. Among these,the CH procedure is most excellent, the RH procedure and the THprocedure are excellent in this order from the view point of obtaining astainless steel having a high spring bending limit.

Besides these three procedures,'a procedure which comprises working asolution heat treated steel at a low temperature of less than C has alsobeen known as a method for transformation of austenite structure insteel into martensite structure. In this invention a transformationtreating step means a step of transforming the solution heat treatedaustenite structure in accordance with. any one of'the above-mentionedvarious procedures into martensite structure.

Accordingly, the characteristics of the present stainless steel isexplained in the following on the basis of the hardening treatmentaccording to theCH procedure.

The Nb to be added to the present stainless steel has a function tolower the hardness of the steel before the final heat treatment, ascompared with the stainless steel having no Nb, and at the same time hasa function to enhance the spring bending limit after the final heattreatment. Y

A preferable range of Nb to be added is O.l3% by weight, but about 1% byweight of Nb is most satisfactory in particular, because, firstly, lessthan 0.15% by weight of Nb does not lower the hardness of a coldrolledsteel before the final heat treatment and thus is not satisfactory;secondly the effect of Nb addition upon enhancing the spring bendinglimit becomes saturated at about 1.4% by weight of Nb; and thirdly theaddition of 3% by weight or more of Nb will not enhance the springbending limit effectively contrary to the expectation of such anincreased proportion of Nb and the steel itself becomes expensive andthe fatigue strength of-the steel is lowered. Thus, these disadvantagesare often encountered in such cases.

Ti has a function almost equal to that of Nb, and thus,

that case, it is desirable that the amount of Ti is one half of theamount of Nb to be replaced.

The Al of the present stainless steel has a function of. enhancing ahardness and a spring bending limit after the final heat treatmentwithout excessively enhancing the hardness before said heat treatment,if added in a proper range.

A preferable range of Al is 01-15% by weight, because in the case thatO.l0.3% by weight of Al is added to the steel, the spring bending limitafter the final heat treatment can be 10 or more enhanced, and even ifinore Al is added, the effect has a tendency of saturation, and additionof 1.5% or more by weight of A1 has no more significance, but theaddition of an increased amount of Al increases a dirtiness of themolten steel, and the solution heat treatment of the steel itself isdifficult. The hardness of steel before the final heat treatment tendsto be abruptly enhanced if the amount of Al to be added exceed 1% byweight. Thus, it is preferable that Al to be added be within a range ofnot more than 1.0% by weight in the case that the workability must betaken into consideration.

Free carbon has a strong action to stabilize the austenitic structureand an action to make worse the corrosion resistance. Such actions canbe somewhat reduced with Nb or Ti to be added in the present invention,because Nb or Ti reacts with said free carbon to fix it in the forms ofsuch compounds as NbC, TiC, etc. However, it is difficult to convert thefree carboncompletely to theform offixed carbon, and thus it is ratherdesirable that the amount of carbon to beadded be 0.1% or less bbyweight. i

Theamount of Si to be added is not so severely restricted as the amountofcarbon, but it is desirable that the amount of Si be 1.4% or less by.weight, because even the addition of 1.4% or more by weight of'Si willrarely help to enhance the spring bending limitof the steel at theprecipitation hardening treatment, but even an almostundetectable-amount of Si can enhance the spring bending limit moresufficiently according to the present invention, than that of theconventional steel.

The present stainless steel contains Mnor unavoidable impurities such asS and P, usually contained in the ordinary stainless steel, in additionto said components.

The amount of said Mn is enough in a range usually used in the ordinarystainless steel to be added to improve its workability, that is, 1.5% orless by weight, and it is needless to say that it is more desirable thatthe amount of other unavoidable impurities is rather less.

Table 1 shows compositions and spring bending limits after the finalheat treatment of the present stainless steel (Examples 1 to 7) andtheconventional 17-7 PH steel (Reference Examples 1 to 3) of thestandard comit is possible to replace a portion of said Nb with Ti. lnpositions. 1

" rams 1 1 Sample Composition (wt. Sprin No. v bending fimit C Si Mn CrN1 A1 Nb Ti After final heat treatment Ref. Ex. 1 008 012 020 180 73 0930- 0 -124 Ref Ex2 009 023 018 167 694 098 0 0 111 Ref. Ex 3 0 09 0 430.62 17.0 7.2 1 03 0 0 125 Table 1Continued In said respective Examplesand Reference Exam When the workability of the 17-7 PH steel must beples, cold rolling is effected at a 50% degree of working after thesolution heat treatment, and the coldrolled steels are heat-treated at480500C for 60 minutes as the final heat treatment, and in Examples 1 to5, the

amount of Al is somewhat made smaller than that of:

the standard composition of the l7-7 PH steel. 1

As is obvious from the Table, addition of about 0.15% by weight of Nb tothe 17-7 PH steel having a standard composition can increase the springbending limit over 160 kg/mm and the spring bending. limit 5 tends to beenhanced according to an increase in the amount of Nb to be added.

Example 6 shows a case where a portion of Nb is replaced with Ti. Inthis case, the increase in the spring bending limit is particularlyremarkable, and a stainless steel having a spring bending limitexceeding 180' kg/mm can be obtained by adding a mixture of about 0.1%by weight of Nb and not more than 0.1% by weight of Ti.

Table 2 shows an example where 1.04% by weight of; Y

Nb is added to a 17-7 PH- steel having the standard composition in whichthe amount of Al is relatively increased, and an example where onlyabout 0.1% of Nbf is added thereto, together with the spring bendinglim-i its of the steels, to which the same precipitation hard-I eningtreatment as in examples of Table l is applied.

taken into consideration, a hardness at the shaping naturally becomesimportant. That is, it can be said that the workability of a steelbecomes better, if the hard- The composition of the stainless steel usedin the Ex- 7 amples is Cr: 17.0% by weight, Ni: 7.0% by weight, Nb: 1.0%by weight, Si: 0.6% by weight, C: 0.1% by weight,-

Mn: 0.4% by weight, Al being selected in a range of 0.05% to 1.4% byweight, and the balance being Fe, and the stainless steels which havebeen cold rolled at a degree of working after the solution heattreatment and heat-treated at 480500C for minutes as the final heattreatmentwere used.

It is evident from the results that the spring bending limit of thestainless steel after the final heat treatment tends to increase inproportion to the amount of Al to be addedwithin a range of 0.1 to 0.3%by weight of Al,

0.3% by weight.

It is clear by comparing the thus obtained result with the result ofExample 3 in said Table 1 that, if the Further,.it can be said thatthehardness before the final heat treatment has a tendency to rapidlyincrease amounts of Nb are equal to each other, that is, about 60 arounda point over 1.0% by weight of Al to be added.

1.0% by weight, a steel having a higher spring bending limit can beobtained in the case that the amount of Al is somewhat larger. Further,it is evident that a steel having much higher spring bending limitcannot be ob- Thus, it can be seen that, in order to obtain a stainlesssteel having a high spring bending limit anda good workability, apreferable amount of Al to be added is within a range of 0.3 to 1% byweight.

tained in the case that the amount of Nb is very small, 5 Table 3 showscomparison of several examples of the for example, 0.1% by weight.

lpresent invention with several reference examples.

TABLE 3 Spring bonding limit Vickers' hardness Composition (wt.percent.) Boim-n Al'tvr lit-torn Attaisan plc timil linnl. llllilllimit. lllml ln-ni. lliutl haul. number Si Mn Cr Ni Al Nb Otherstrnntnmnt. trnntmon l. troutmon I. tronttnnnt.

0. 53 0. 32 16. 76 7. 02 0. 39 1. 61 101 340 483 0. 52 0. 43 16. 75 6.87 0. 79 L0] 55 180 346 521 0. 55 0. 52 16. 77 7. 02 0. 21 1.05 62 178341 476 0. 62 0. 58 16. 92 6. 86 0. 19 0. 42 70 169 397 483 O. 64 O. 5416. 79 6. 82 0. 1. 44 49 181 353 498 1. 08 0.27 17.32 7. 52 O. 73 1.0457 194 362 519 0. 02 O. 23 17. 81 7. 43 0. 72 0.97 52 182 367 521 1. 040. 05 16. 33 6. 91 0. 74 0. 50 53 200 377 502 O. 07 0. 28 14, 93 6. 850. 75 1. 05 55 180 380 520 0.35 0. 45 16. 55 6. 95 0. 40 3. 02 51 167393 530 0. 07 0. 29 12. 53 6. 75 0. 73 0. 95 60 165 390 540 O. 53 0. 7417. 66 6. 93 1. 40 1. 07 99 179 43} 603 0. 53 0. 37 16. 77 6. 94 0 115126 53 3 546 0. 52 0. 40 16. 74 6. 93 0. 89 47 111 488 597 0. 62 1. 1418. 43 8. 53 0 61 114 404 367 0. 70 0. 16. 86 10. 80 0 63 122 354 373 0.53 0. 69 16. 75 7. 03 0 65 163 342 466 The stainless steels used in theExamples and Refer ence Examples shown in Table 3 were sheet materialscold-rolled at a 50% degree of working after the solution heat treatmentand subjected to the final heat treatment.

As the final heat treatment, a heat treatment was applied at 600C for 60minutes in Reference Example l6, and at 480,500C for 45 minutes in otherExamples and Reference Examples.

In Reference Example 14, a kind of well-known stainless steels forspring purpose, which have an unstable austenitic structure after thesolution heat treatment, but are of non-precipitation hardeningmaterial, though a martensitic structure is developed by cold working.The steel of this Reference Example has such a difficulty that thehardness after the cold rolling is very high, and has not-so high springbending limit after the heat treatment as that of the present stainlesssteel.

The stainless steels of Reference Examples 16 and I? have theconventional stable austenitic structure. In these cases, the hardnessof cold-rolled steel can be lowered to some degree, but the springbending limit.

after the precipitation hardening treatment cannot be made excessivelyhigher.

Example 21 shows a case where the amount of Al is rolling and a springbending limit of 180 kg/mm after,

the final heat treatment can be readily obtained.

As shown in Examples 18 and 20, the stainless steel, a portion of whoseCr is replaced with Mo, have also a high spring bending limit. Byreplacing a portion of Cr with M0 in this way, a stainless steel havinga good high temperature .strength, though its hardness is somewhatenhanced, can be obtained.

As explained above, it has been found, as shown in the examples, that astainless steel having a low hardtioned. The spring is classified intotwo cases according to the method for holding the shadow mask. That is,the one is a case where a material of a low thermal expansion isrequired, and the other is a case where a material having a coefficientof thermal expansion close to that of the material constituting theshadow mask (usually pure iron is used) is required.

it is the latter case that the present stainless steel is used as aspring for said holding purpose. In the latter spring case, a materialhaving an austenitic structure is not appropriate, because of itsexcessively high coefficient of thermal expansion, and a material havinga martensitic structure, as in the present stainless steel, is required.

In order to avoid an occurrence of a displacement of the shadow maskposition due to a vibration or shock, it is necessary to use a springhaving a high resistance to the so called relaxation. In this regar d,the present stainless steel having a high spring bending limit canperform an excellent service.

Further, it is required in mass production of springs to conduct shapingreadily by means of press punching and save a loss of pressdie. In thisregard, the present stainless steel is excellent, because the hardnessat the shaping, that is, before the precipitation hardening treatment islow.

What is claimed is:

1. A martensitic precipitation-hardening stainless steel capable ofbeing transformed to a substantially martensitic structure by coldworking which consists essentially of Y 6.50 7.75% by weight of Ni,

l6] 8% by weight of Cr,

0.l5l.5% by weight of Nb,

01-10% by weight of A],

not more than 0.1% by weight of C,

not more than l.4% by weight of Si,

not more than l.5% by weight of Mn, and the balance being unavoidableimpurities and Fe,

6.50 7.75% by weight of Ni,

16- 18% by weight of Cr,

0.15 1.5% by weight of Nb,

0.1 1.0% by weight of Al,

not more than 0.1% by weight of C, not more than 1.4% by weight of Si,not more than 1.5% by weight of Mn,

and the balance being unavoidable impurities and Fe,

the proportions of Ni and Cr being so selected and arranged that asubstantially transformable austenitic structure or a considerableamount of retained austenitic structure prevails after solution heattreatment. and

said austenitic structure is transformed to said substantiallymartensitic structure when a cold working is applied thereto.

4. The martensitic precipitation-hardening stainless steel according toclaim 3, wherein the amount of Si is in the range of 0.021% by weight.

5. A martensitic precipitation-hardening stainless steel capable ofbeing transformed to a substantially martensitic structure by a coldworking which consists essentially of 6.50 -7.75% by weight of Ni, 1618% by weight of Cr, 0.15 1.5% by weight of Nb, 0.1 1.0% by weight ofAl, not more than 0.1% by weight of C, not more than 1.4% by weight ofSi, not more than 1.5% by weight of Mn, and the balance beingunavoidable impurities and Fe, with a portion of Nb being replaced withone-half of the portion of Ti, and the proportions of Ni and Cr being soselected and arranged that a substantially transformable austeniticstructure or a considerable amount of retained austenitic structureprevails after solution heat treatment, and said austenitic structure istransformed to said substantially martensitic structure when a coldworking is applied thereto.

6. -A martensitic precipitation-hardening stainless martensiticstructure by a cold working which consists essentially of 6.50 -7.75% byweight of Ni, 16 18% by weight of Cr, ().l5%1.5% by weight of Nb, 0.11.0% by weight of Al, not more than 0.1% by weight of C. not more than1.4% by weight of Si, not more than 1.5% by weight of Mn, the thebalance being unavoidable impurities and Fe, with a portion of Cr beingreplaced with the same amount of Mo, and the proportions of Ni and Crbeing so selected and arranged that a substantially transformableaustenitic structure or a considerable amount of retained austeniticstructure prevails after solution heat treatment and said austeniticstructure is transformed to said substantially martensitic structurewhen a cold working is applied thereto.

7. A martensitic precipitation-hardening stainless steel capable ofbeing transformed to a substantially martensitic structure by a coldworking which consists of 6.50 7.75% by weight of Ni, 16 18% by weightof Cr, 0.15 1.5% by weight of Nb, 0.] 1.0% by weight of Al, not morethan 0.1% by weight of C, not more than 1.4%.by weight of Si, not morethan 1.5% by weight of Mn, and the balance beingunavoidable impuritiesand Fe, with a'portion of Nb being replaced with one-half of the portionof Ti, and the proportions of Ni and Cr being'so selected and arrangedthat a substantially transformable austenitic structure or a considerable amount of retained austenitic structure prevails after solutionheat treatment, and said austenitic structure is transformed to saidsubstantially martensitic structure when a cold workingisappliedzthereto.

8. A martensitic precipitation -hardening stainless steel capable ofbeing transformed to a substantially martensitic structure by a coldworking which consists of 6.50 7.75% by weight'of Ni, 16 18% by weightof Cr, 0.15 l.5% by weight of Nb, 0.] 1.0% by weight of Al, not morethan 0.1% by weight of C, not more than 1.4% by. weight of Si, not morethan 1.5% by weight of Mn, and the balance beingunavoidable impuritiesand Fe, with a portion of Cr beingreplacedwith the same amount of Mo,and the proportions of Ni and Cr being so selected and arranged that asubstantially transofrmable austenitic structure or a considerableamount of retained austenitic structure prevails after solution heattreatment and said'austenitic structure is transformed to saidsubstantially martensitic structure when a cold working is appliedthereto.

steel capable of being transformed to a substantially

1. A MARTENSITIC PRECIPITATION-HARDENING STAINLESS STEEL CAPABLE OFBEING TRANSFORMED TO A SUBSTANTIALLY MARTENSITIC STRUCTURE BY COLDWORKING WHICH CONSISTS ESSENTIALLY OF 6.50-7.755 BY WEIGHT OF NI, 16-18%BY WEIGHT OF CR, 0.15-1.5% BY WEIGHT OF NB, 0.1-1.0% BY WEIGHT OF AL,NOT MORE THAN 0.1% BY WEIGHT OF C, NOT MORE THAN 1.4% BY WEIGHT OF SI,NOT MORE THAN 1.5% BY WEIGHT OF MN, AND THE BALANCE BEING UNAVOIDABLEIMPURITIES AND FE, THE PROPORTIONS OF NI AND CR BEING SO SELECTED ANDARRANGED THAT A SUBSTANTIALLY TRANSFORMABLE AUSTENITIC STRUCTURE OR ACONSIDERABLE AMOUNT OF RETAINED AUSTENITIC STRUCTURE PREVAILS AFTERSOLUTION HEAT TREATMENT, AND SAID AUSTENTIC STRUCTURE IS TRANSFORMED TOSAID SUBSTANTIALLY MARTENSITIC STRUCTURE WHEN A COLD WORKING IS APPLIEDTHERETO.
 2. The martensitic precipitation-hardening stainless steelaccording to claim 1, wherein the amount of Si is in the range of0.02-1% by weight.
 3. A martensitic precipitation-hardening stainlesssteel capable of being transformed to a substantially martensiticstructure by cold working which consists of 6.50 - 7.75% by weight ofNi, 16- 18% by weight of Cr, 0.15 - 1.5% by weight of Nb, 0.1 - 1.0% byweight of Al, not more than 0.1% by weight of C, not more than 1.4% byweight of Si, not more than 1.5% by weight of Mn, and the balance beingunavoidable impurities and Fe, the proportions of Ni and Cr being soselected and arranged that a substantially transformable austeniticstructure or a considerable amount of retained austenitic structureprevails after solution heat treatment, and said austenitic structure istransformed to said substantially martensitic structure when a coldworking is applied thereto.
 4. The martensitic precipitation-hardeningstainless steel according to claim 3, wherein the amount of Si is in therange of 0.02-1% by weight.
 5. A martensitic precipitation-hardeningstainless steel capable of being transformed to a substantiallymartensitic structure by a cold working which consists essentially of6.50 -7.75% by weight of Ni, 16 - 18% by weight of Cr, 0.15 - 1.5% byweight of Nb, 0.1 - 1.0% by weight of Al, not more than 0.1% by weightof C, not more than 1.4% by weight of Si, not more than 1.5% by weightof Mn, and the balance being unavoidable impurities and Fe, with aportion of Nb being replaced with one-half of the portion of Ti, and theproportions of Ni and Cr being so selected and arranged that asubstantially transformable austenitic structure or a considerableamount of retained austenitic structure prevails after solution heattreatment, and said austenitic structure is transformed to saidsubstantially martensitic structure when a cold working is appliedthereto.
 6. A martensitic precipitation-hardening stainless steelcapable of being transformed to a substantially martensitic structure bya cold working which consists essentially of 6.50 -7.75% by weight ofNi, 16 - 18% by weight of Cr, 0.15%-1.5% by weight of Nb, 0.1 - 1.0% byweight of Al, not more than 0.1% by weight of C, not more than 1.4% byweight of Si, not more than 1.5% by weight of Mn, the the balance beingunavoidable impurities and Fe, with a portion of Cr being replaced withthe same amount of Mo, and the proportions of Ni and Cr being soselected and arranged that a substantially transformable austeniticstructure or a considerable amount of retained austenitic structureprevails after solution heat treatment and said austenitic structure istransformed to said substantially martensitic structure when a coldworking is applied thereto.
 7. A martensitic precipitation-hardeningstainless steel capable of being transformed to a substantiallymartensitic structure by a cold working which consists of 6.50 - 7.75%by weight of Ni, 16 - 18% by weight of Cr, 0.15 - 1.5% by weight of Nb,0.1 - 1.0% by weight of Al, not more than 0.1% by weight of C, not morethan 1.4% by weight of Si, not more than 1.5% by weight of Mn, and thebalance being unavoidable impurities and Fe, with a portion of Nb beingreplaced with one-half of the portion of Ti, and the proportions of Niand Cr being so selected and arranged that a substantially transformableaustenitic structure or a considerable amount of retained austeniticstructure prevails after solution heat treatment, and said austeniticstructure is transformed to said substantially martensitic structurewhen a cold working is applied thereto.
 8. A martensitic precipitation-hardening stainless steel capable of being transformed to asubstantially martensitic structure by a cold working which consists of6.50 - 7.75% by weight of Ni, 16 - 18% by weight of Cr, 0.15 -1.5% byweight of Nb, 0.1 - 1.0% by weight of Al, not more than 0.1% by weightof C, not more than 1.4% by weight of Si, not more than 1.5% by weightof Mn, and the balance being unavoidable impurities and Fe, with aportion of Cr being replaced with the same amount of Mo, and theproportions of Ni and Cr being so selected and arranged that asubstantially transofrmable austenitic structure or a considerableamount of retained austenitic structure prevails after solution heattreatment and said austenitic structure is transformed to saidsubstantially martensitic structure when a cold working is appliedthereto.